398 navi:n^ on the horse. 



I have thus described the general outlines of the distribution 

 of the blood-vessels, but there are thousands of twigs of arteries 

 and veins I have not mentioned. I have spoken of three kinds 

 of blood-vessels — the arteries and veins, and the fine net-work 

 of capillaries (hair-like vessels) connecting them. It remains 

 to point out the uses of the circulation of the blood, for which 

 these vessels are designed. But, before proceeding to this sub- 

 ject, I will remark that the heart, which is the central organ of 

 the circulation of the blood, is subject to several diseases, as 

 softening of its walls, thickening of the same, inflammation, 

 both of itself and investing sack. 



The large arteries near the heart, and also their valves, are 

 liable to become changed into bone or cartilage. The walls of 

 arteries sometimes become partially broken, giving rise to large 

 pulsating, or beating, tumors, called aneurisms. 



The veins are liable to inflammation, which is sometimes 

 caused by bleeding. They may become ruptured or broken. 

 They are also liable to become enlarged, as in blood-spavin, etc. 



Both arteries and veins may be cut by accident. If an 

 artery is cut, the blood flows in jets, and is of a bright color ; 

 if a vein, the blood runs in an even stream, and is of a dark 

 color. (For proper treatment seethe article on "Wounds.") 



CIRGULATIOX OF THE BLOOD. 



"Of the flesh the blood is the life thereof." It contains 

 every thing that is necessary for building up and repairing 

 every part of the animal body, from the hardest bones to the 

 softest parts of the system. It is in the form of a fluid of a 

 red color. Its redness depends on what are called the red cor- 

 puscles. As the blood circulates through the capillaries, it 

 loses the oxygen which it took up in the lungs, and becomes 

 charged with carbonic acid gas and other impurities, and 

 changed to a dark color. It is then carried on to the heart by 

 the veins, and from there sent to the lungs, where the car- 

 bonic acid, and some other impurities, are given oif, and the 



